Slurries of papermakers"" pulp are conventionally screened in pressure screening apparatus in which moving foil members rotate in closely spaced relation to a screen inlet surface, and apply to such surface positive and negative pressure pulses that tend to keep the screen openings or slots clear and free from blocking by fiber mat build up on the screen surface. Two or more foils are commonly rotated at surface speeds as high as 5,000 feet per minute. Such conventional screening apparatus has been highly successful and is in widespread use in pressure screens, and examples may be seen in any one of the following U.S. Patents and the references cited in such patents: U.S. Pat. Nos. 5,385,240, 4,919,797, 5,078,275, 4,267,035 and 4,166,028.
Such rotating foil screening apparatus requires substantial energy to rotate the foils. However, such screens are inherently subject to failure due to the complexity of the construction, and the necessity to maintain a rotary structure and mechanism to drive the rotary structure at the desired speed and power input rate. There is accordingly a need in the industry to provide a screen that utilizes existing screening elements and concepts, such as the flat or the cylindrical basket type screen, and in which the rotary elements are reduced or eliminated altogether and by which the reliability is increased, and capital expenditure and energy demands are substantially decreased.
This invention provides a screen and methods for screening papermakers"" pulp that fluidize the fiber mat forming on the inlet side of the screen surface with comparatively low energy requirements. In one embodiment this is accomplished by periodically varying the pressure on the inlet side of the screen at a given frequency and simultaneously varying the pressure at the outlet side of the screen, with a pressure waveform in an out-of-phase condition with a pressure waveform on the inlet side of the screen, such that the differential pressure across the screen periodically reverses in polarity. Accordingly the fiber mat tending to form at the screening element inlet surface is disrupted by the periodic or momentary reverse flow conditions that occur during the pressure reversals.
The mechanism by which the flow reversals are created, and the rate or frequency thereof, may be varied depending upon the requirements for the particular equipment, the rate of mat formation, the consistency of the slack slurry, etc. Flow reversal through the screen openings or slots may be caused through a mass momentary flow reversal of the content of the screen, but need only be localized at the screen slots or openings. The use of phased sonic or ultrasonic transducers positioned to direct a waveform output simultaneously to the inlet and outlet surfaces, or the inlet surfaces alone, to disrupt the mat formation is considered to be within the scope of this invention.
In an embodiment illustrated in the drawings, sychronized pulsating tranducers or generators in the form of pumps, one pump at the inlet feed and pump at the accepts outlet, are controlled to effect periodic flow reversals at the screen. Also, an oscillating piston type of pump or equivalent could be connected with one end at the inlet and another end at the outlet and driven at a desired rate to effect the desired inverted pressure waveforms at the inlet and outlet surfaces of a screen.
In a broad sense, the method is independent of the particular waveforms of the pressure at the inlet screen and the pressure at the outlet screen, i.e., such pressure waveforms need not be sinusoidal as long as periodic momentary flow reversals or pressure waves disrupt the formation of a fiber mat on or at the screen inlet surface. In a cylinder type screen, either the inner surface or the outer surface may be the inlet surface, depending upon the flow arrangement of the pressure screen.
In the broadest sense, these concepts and methods are independent of screen geometry such as the shape or nature of the screening components themselves or that of the feed or accepts chambers. They are also independent of the specific means by which the synchronized and opposite pressure changes are created or applied across the screen.
It is accordingly an important object of the invention to provide simplified variable pressure screening apparatus and method in which flow reversals are induced across a screen by oppositely and cyclically varying the fluid pressure at (or associated with or integrated into) a screen inlet and screen outlet.
A further object of the invention is the use of pulsating pumps or pressure transducers that are synchronized to cause periodic flow reversals across a screen element in a pressure screen or papermakers"" pulp.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.